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Photolysis of Cyclopentadienide and Cyclopentadiene Eugene E. van Tamelen, John I. Brauman, and Larry E. Ellis Contribution from the Department of Chemistry, Stanford University, Stanford, California 94305. Received September 30, 1970 Abstract : In the first study of product formation from an excited carbanionic species, sodium cyclopentadienide in tert-butyl alcohol-tetrahydrofuran was found on ultraviolet irradiation to produce small yields of meso- and dl-343 '-cyclopenteny1)cyclopentenes (2a, b). Mechanistic studies suggest that these two products are formed from cyclopentenyl radicals, produced by protonation of radical anions generated by abstraction of hydroxylic hydrogen atoms of tert-butyl alcohol by excited cyclopentadienide. Ultraviolet irradiation of cyclopentadiene in neutral or weakly basic alcoholic solution leads to formation of the valence bond isomer, bicyclo[2.1.O]pent-2-ene(4), the smallest known unsaturated polycyclic hydrocarbon. The new isomer was characterized spectrally and was observed to undergo: (1) diimide reduction to bicyclo[2.1.O]pentane, (2) a Diels-Alder reaction with cyclopentadiene, and (3) thermal isomerization to cyclopentadiene. Theoretical aspects are discussed.
D
espite the great variety and considerable number of organic substances which have been subjected t o photochemical investigation, there have appeared no reports on the nature of products formed during ultraviolet irradiation of carbanions, a well-known and useful species in organic chemistry. In initiating work in this area, we have concentrated our attention thus far on the behavior of excited carbanions of the aromatic type, partly because of ( I ) their high order of groundstate stability and (2) the presence of chromophores well suited for irradiation and possible conversion t o valance bond isomeric systems or other interesting products. The first true anionic case to be studied was cyclopentadienide (1).
@ 1
Since the planning of experimental details had in this study distinct ramifications in the entire body of chemistry which developed, we comment first on the reasons for selecting certain procedures used. In the absence of a proton source, the immediate product deriving from excited cyclopentadienide would likely be in itself a carbanion, probably corresponding t o an acid considerably weaker than the original cyclopentadiene. In order t o trap the anionic photoproduct and thereby forestall reversion t o cyclopentadienide or formation of some still different transformation product, protonation immediately following chemical reaction of the excited cyclopentadienide seemed desirable. However, inclusion of an acid of the strength appropriate t o ensure such protonation complicates the irradiation system in the sense that the presence of some amount of cyclopentadiene in equilibrium with the anion might now be permitted, with resultant ambiguity as t o the true source of isolated final products. In order t o remove absolutely such doubts, identification of products from bofh hydrocarbon and derived carbanion was made, the photoreactions being carried out under conditions closely similar, except for level of basicity. In consequence, not only were products definitely derived from excited anion isolated, but also formation of a novel valence bond isomer was shown t o be a result of cyclo(1) Preliminary communications: (a) E. E. van Tamelen, L. E. Ellis, and J. I. Brauman, J . Amer. Chem. SOC,89,5073 (1967); (b) J. I. Brauman, L. E. Ellis, and E. E. van Tamelen, ibid., 88, 846 (1966).
pentadiene irradiation under the particular conditions used in these studies. The ultraviolet irradiation of sodium cyclopentadienide in 20 % tert-butyl alcohol-80 % tetrahydrofuran gave in approximately 5 % total yield equal amounts of the meso- and dl-3-(3 '-cyclopenteny1)cyclopentenes (2a, b) (identified by vpc, nmr, and mass spectral comparison with authentic specimens).* The photolysis was car-
moa 2a, b
3
meso and d, 1
ried out with a Hanovia quartz immersion well apparatus fitted with a 450-W Hanovia high-pressure mercury arc lamp. After completion of the reaction, volatile materials were analyzed by vpc methods, and nonvolatile products by column chromatography. In addition t o the dicyclopentenyls, small amounts of unidentified colored hydrocarbons as well as considerable polymeric material were formed. Bicyclo[2.1 .O]pent-2ene (4) was not formed under these conditions; however, when excess cyclopentadiene was present, bicyclopentene was produced. Light transmission of the solution decreased rapidly during photolysis as a dark color developed, and the irradiated solution became dark brown on exposure t o air or water. None of these substances was observed after work-up of the unirradiated solution of the anion. Carbon skeletons of the 3,3'-dicyclopentenyls were established by catalytic hydrogenation and diimide reduction t o dicyclopentyl. The degree of unsaturation was found by mass spectral analysis of each photoproduct and of the dicyclopentyl-d4 obtained by deuteriodiimide reduction. The mass spectra of photoproduct 2a and 2b were identical, with the major fragment having a mass of 67. Controlled diimide reduction of the mixture of photoproducts resulted in a single partial reduction product, 3-cyclopentylcyclopentene (3). The photoproducts Za and b exhibited only end absorption (2) For our purposes, authentic meso- and dl-3-(3'-dicyclopenteny1)cyclopentene were prepared by Grignard coupling of 3-chlorocyclopentene. The hydrocarbons were obtained previously by the benzophenone-sensitized photolysis of cyclopentene, presumed to generate cyclopentenyl radicals which couple to observed products: I
